Design of A Pentagon Microstrip Antenna for Radar Altimeter

International Journal of Web & Semantic Technology (IJWesT) Vol.3, No.4, October 2012

DOI : 10.5121/ijwest.2012.3404 31

Design of A Pentagon Microstrip Antenna forRadar Altimeter Application

K. RamaDevi1, A. Mallikarjuna Prasad2 and A. Jhansi Rani3

1 ECE Dept., Pragati Engg. College, Kakinada, A.P., India2 ECE Dept., JNTU College of Engineering, Kakinada, A.P., India

3 ECE Dept., V.R. Siddhartha Engg. College, Vijayawada, A.P., India

{kolisettyramadevi, a_malli65}@yahoo.Com, Jhansi9rani@gmail.com

Abstract

In the navigational applications, radar and satellite requires a device that is a radar altimeter. Theworking frequency of this system is 4.2 to 4.3GHz and also requires less weight, low profile, and high gainantennas. The above mentioned application is possible with microstrip antenna as also known as planarantenna. In this paper, the microstrip antennas are designed at 4.3GHz (C-band) in rectangular andcircular shape patch antennas in single element and arrays with parasitic elements placed in H-planecoupling. The performance of all these shapes is analyzed in terms of radiation pattern, half power points,and gain and impedance bandwidth in MATLAB. This work extended here with designed in different shapeslike Rhombic, Pentagon, Octagon and Edges-12 etc. Further these parameters are simulated in ANSOFT-HFSSTM V9.0 simulator.

KeywordsRectangular patch, Circular Patch, Pentagon shape, impedance bandwidth, radio altimeter etc.

1. Introduction

A radio altimeter is a device, which is used to measure a low altitude or distance from an aircraftor spacecraft to ground surface or to a sea level. This distance is calculated under the craft invertical direction. Radio altimeter is a part of radar. The working principle of radar is, it transmitsradio waves towards ground level or sea level and receives an echo signal after time duration.This value of time is depending on speed of the vehicle and height between craft (air or space)and ground. Here an antenna places a pivotal role to transmit the radio waves and receiving ofwaves either at the same frequency or at a band of frequencies. The working principle of a radioaltimeter is shown in Fig1. Here two antennas are used, one for transmitting radio waves andother for receiving an echo signal reflected by ground or surface of terrains. The receiving time isa ratio of two times of altitude between craft and reflected region to light velocity in space.Generally, frequency modulated continuous wave is preferred than simple continuous wavetechnique [1]. Radio altimeter [1-9] is working in the band of 4.2GHz to 4.4GHz [2] andreceiving frequency is differed from transmitted signal at a rate of 40Hz per foot. The altimetersare also used in both civil and military aircrafts. In civil applications these are used at lowvisibility conditions and automatic landing systems. Civil applications altimeters give thereadings up to 2,500 feet and weather altimeters give up to 60,000feet above the ground level(AGL). Military applicator altimeter is used to fly quite low over the land and the sea to avoid

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radar detection and targeting by anti-aircraft guns or surface to air missiles.

Fig: 1 Basic Conceft of Radio Altimeter

The apt antenna for radio altimeter is a microstrip antenna and also called as patch antenna [10-11]. They are also used in the millimeter-wave frequency range. Microstrip patch antennaconsists of a patch of metal that is on top of the grounded dielectric substrate of thickness h,with relative permittivity and permeability εr and μr (=1) as shown in Fig.2a &2b. Themetallic patch may be of various shapes with rectangular, circular, and triangular etc.

The general single antennas offer low gain and low directivity. These values are not suitable forvarious areas like cellular, mobile, satellite communication, navigation, biomedical etc. and theseparameters are to be improved. The most popular method is an array i.e., arrangement of similarpatches in different structures like linear, ring, cross etc. The scientists YAGI and UDA observedand suggested the designing parameters like distance between different patches [10]. Thesepatches are named as reflector, driven and directors. Along with these parameters the patch sizecan be changed for effective results. This well known method is named after these scientists asYagi-Uda array.

Antenna array has large number of elements and has several problems if microstrip antennaelements are used. First, if each element is connected to a feed line, the resulting feeding networkwill introduce unwanted radiations as a result the copper losses [10-11]. Second, for phase array,each individual element will require a phase shifter in order for the beam to be steered, with resultthat a great number of phase shifters are needed in large arrays; the cost of the phase shifters islikely to be increased. K.F.Lee et.al[12] suggested that problem will be reduced if the array isdivided in to sub arrays and feeding is given to only one patch in each of the sub array [12] , with

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several closely spaced parasitic patches (called Reflector, and Directors) around it. The elementthat is directly fed is called driven element.J.Huang [13] presented the work on rectangular patch yagi-array antenna. Parasitic patches arearranged linearly as shown in Fig.2 .He designed an antenna at 6.9GHz (C-band) and 1.58 GHz(L band), for linear polarization. In his observations the beam tilted towards end-fire direction. Hecontinued his work for circular polarization too. D.P.Gray, et al [17] studied on linearly polarizedyagi-array at L-band (1-2GHz) with 4-elements and also in S-band (2 - 4GHz) with six elements.He extended his work with different substrate materials and heights. In the later years, researcherscontinued the similar topic. Yang-Chang et al [14] continued their work on rectangular patchlinearly polarized yagi-array antenna at 38GHz and observed response with different sizes anddistance between elements. Chow Yen Desmond Sim et al [18] presented the work on Annular–ring microstrip patch for circular polarization at S-band frequency range. Garima et al [16]simulated the work on circular patch in C-band at 6GHz and observed low directivity with singlepatch.

Krishna Kumar et.al[5], has done work on Octagonal microstrip antenna for Radar and Space-Craft Applications. He had designed antenna in HFSS and observed results.

Before designing, the practical antenna can simulate using softwares like ANASOFT-HFSSTM

V9.0 [23] and MATLAB. By simulation antenna characteristics can be analyzed and synthesized.These characteristics can be visualized in all dimensions Mr. M.Ben Ahmed et al., [19-24]designed a patch antenna for multi applications like GSM/PCS/UMTS/HIPERLAN for MobileCellular Phones in softwares. Later he tested practically and observed that both results are similar.

2. Design Procedure

In this paper Yagi antenna at 4.3GHz (C band) in linear structure with both rectangular andcircular patches (Fig. 3) having linear polarization [13] is designed. Further, work was extendedto different shapes [5] of patches like Square, Rhombic, Pentagon, Octagon and Edge-12 which isnearer to circular structure. All these structures are designed in ansoft HFSS simulator [23]which is working in finite discrete time domain(FDTD) and analyzed in terms of percentageimpedance bandwidth and gain in dB on both E-Plane and H-plane.

For Rectangular Patch Antenna:

Width of the patch W= [C/2*fr]*sqrt{2/(εr+1)} --- (1)Where C is velocity of light in free spacefr is a resonant frequency or operating frequency

Length of the patch ‘L’ is generally λo/3 <L < λo/2

Circular Patch Antenna:

--- (2)where

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--- (3)The percentage bandwidth [11] is calculated from return loss (S11) graph of antenna below10dB.%Impedance bandwidth = 200 ∗ ( )( ) ----- (4)

Where fh and fl are upper and lower frequency values from return loss.The array structure shown in Fig.3 is designed with RT-Duroid material as substrate and assumedand calculated values are given in Table-1.

Table: 1

Relative Permittivity εr = 2.2Height h=0.1588cmRectangular patch as driven element Width(w)=2.7578cm, Length(L)= 2.4296cmCircular patch as driven element Radius ‘ a’= 1.2874cmReflector size Driven Element size increased by 0.3cmDirector size Driven Element size decreased by 0.1cmDistance between reflector anddriven element

D1 0.2 λ

Distance between director1 anddriven element

D2 = 0.4λ

Distance between director2 anddirector1

D3 = 0.8λ

Distance between director3 anddirector2

D4 = 0.05λ

Distance between director3 anddirector4

D5 = 0.05λ

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3. Results

With the values obtained in section2, arrays with different combinations 2-directors (red line inFig.6), 3-directors (green line in Fig.6), and 4-directors (blue line in Fig.6) are designed andsimulated in MATLAB. The simulated values of single element are tabulated in Table-2 andpattern on E-Plane and H-Plane is shown in Fig.4.

By keeping the distance between the elements constant and varying the frequency form 1GHz to20GHz (includes L-band, S-band, C-band, X-band, Ku-band and K-band) responses arecalculated and tabulated in Table-3 . From the response with 2-directors, it is observed that

• Up to 8GHz no side lobes are presented.• At 4.3GHz the response on E-Plane of a rectangular patch array is shown in Fig.5, H-Plane is

shown in Fig.7 and E-plane of circular patch array is shown in Fig.6.• Here directivity is around 14.38dB and it is higher than single patch element and beam steered

is 32.650 in end-fire direction.• As frequency increased, directivity also increased and from 8GHz onwards side lobes are

generated in circular patch array. Response at 15GHz is shown in Figs (8-9).• By Keep frequency constant and vary the distances between elements and response tabulated in

Table-4.• The directivity is decreased as permittivity (εr) is increased and tabulated in Table-5.• The height (h) and permittivity (εr) both are simultaneously changed and response is tabulated

in Table-6.• The rectangular patch array antenna at 4.3GHz is designed and simulated in ANSOFT-HFSS is

shown in Fig.10.• The return loss (S11) is shown in Fig.11 represents, it resonates at two frequencies, one is at

3.93GHz with -10dB and second is at 7.17GHz with -33dB.• The directivity of an array in dB (shown in Fig. 12) in φ = 0odeg plane shows 12.27dB with

49.90 tilt towards end fire direction.• Feed tuning in rectangular patch and array is difficult than other structures like circular, 5 edges

( pentagon) and more.• Referring to the Fig.13 the radius of a pentagon is 1.377cm which is designed in ansoft HFSS .• From Fig.14 the return loss that replicates is indicated with S11. It is tuned at the resonating

frequencies FR1=4.36GHz and FR2= 9.09GHz respectively and also called dual tunedantenna. It is suitable for both radio altimeter application and space craft applications.

• Fig.15 implies that FR1 offers 100MHz bandwidth from 4.21GHz to 4.31GHz below -10dB asrepresented in the S11 graph.

• It produces a gain of 7.44dB with main beam along 0 deg direction as shown in Fig.16.• Fig.17 resembles FR2 that offers maximum gain of 6.57dB and main beam is tilted with

52deg towards end- fire .• FR2 offers a bandwidth of 730MHz .The % impedance bandwidth corresponding to FR2 is 7.8

as tabulated in Table 7.• Fig.18 resembles the current distribution of the pentagon patch which is referred in Fig.13.• Fig. 19 shows the 3D representation at FR1.Similarily the 3D representation is in Fig.20.• The above criterion are repeated for different shapes like Rhombic, Square, Octagon, Edge-12

etc and is shown in Table-7.

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Table:2 Comparison between Rectangular and Circular Single Patch Antenna

Shape of apatch

E-plane(HPBW)

(deg)

H-Plane(HPBW)

(deg)

E-PlaneDirectivity

(dB)

H-PlaneDirectivity

(dB)Rectangular

Shape85 80 7.566177 8.092756

CircularShape

155 1102.347921

5.326702

Table 3: Response of an Antenna w.r.to Frequency

Rectangular -Yagi Antenna with 2-Directors

Frequency(GHz)

Shiftangle(Til

t)(deg)

E-field(dB)

HPBW

(deg)

Directivity(dB)

1 33.29 6.604 41.26 13.843972 33.94 6.674 40 14.113363 33.32 6.745 40 14.11336

3.5 32.06 6.767 40 14.113364 33.29 6.809 38.76 14.38688

4.3 32.65 6.824 38.76 14.386886 33.1 6.922 38.76 14.38688

10 31.78 7.094 37.5 14.6739315 23.99 7.208 35 15.2731925 17.98 7.104 30 16.61213

Circular patch-Yagi Antenna with 2-Directors

Frequency

(GHz)

ShiftAngl

e(deg)

E-field(dB)

HPBW(deg)

Directivity(dB)

3.5 34.49 3.295 38.34 14.481514.3 35.37 3.903 38.34 14.481515 34.42 4.366 38.34 14.481516 37.71 5.001 36.66 14.87071

8 27.095.994

31.66 16.144SLL=1.

27710 25.91 6.774 31.66 16.144

15 24.497.717

25 18.195SLL=3.

057

20 36.577.524

21.66 19.441SLL=0.

863

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Table 4: Response of an Antenna w.r.to distance between elements

Distance(λ)

Tilt(deg)

E (dB) HPBW(deg)

Directivity(dB)

Distance between Reflector and Driven Element(D1)

0.1 31.31 4.664 20 14.11336

0.2 32.65 6.824 19.38 14.38688

0.3 33.88 7.891 19.38 14.38688

Distance between Director and Driven ElementD2(λ),D3(λ)0.3,0.8 35.8

7.70157.5 10.9612

0.4,0.8 37.13 1.875 38.76 14.38688

Table 5: Response of an Antenna w.r.to SUBSTRATE (εr)Rectangular patch -Yagi Antenna with 2-Directors,h=0.1588 cmFrequency

(GHz)εr

Tilt(deg)

E(dB)

HPBW(deg)

Directivity(dB)

4.3 2.2 32.65 6.824 38.76 14.38688

4.3 2.34 33.1 6.62 38.76 14.38688

4.3 3.5 32.79 5.282 42.5 13.58678

4.3 4.4 31.1 4.629 47.5 12.62068

4.3 6.15 31.09 3.997 86.26 7.438366

4.3 10.2 30.83 3.93 103.76 5.833956

Circular patch -Yagi Antenna with 2-Directors

Frequency(GHz)

εrTilt

(deg)E

(dB)HPBW(deg)

Directivity(dB)

4.3 2.2 32.65 6.824 38.76 14.38688

4.3 2.34 35.71 3.702 38.34 14.48151

4.3 3.5 33.07 2.592 51.66 11.89147

4.3 4.4 33.27 2.103 51.66 11.89147

4.3 10.2 32.73 1.25 110 5.326702

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Table 6: Response of an Antenna w.r.to Substrate and Height

Table: 7 Response of different Shapes of patch antenna

Name of an antennaStructure

ImpedanceBand

Width(%)

FR1(GHz)

Gain(DB)

FR2(GHz)

Gain(DB)

Rectangle 1.72 3.82 6.94 ---- ---

Square 5.1 3.91 6.823 7.73 5.89

Circle 8.84 4.39 7.355 9.27 5.58Rhombic 5.81 4.62 7.63 9.19 4.87

Pentogon 7.8 4.36 7.44 9.09 6.57

Octagon 6.89 4.03 7.05 8.64 5.07

Edges12 5.45 4.09 7.10 8.36 4.31

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4. Conclusion

The design of rectangular patch Yagi antenna and circular patch Yagi antenna that operates atlinear polarization has been done using MATLAB, ANSOFT-HFSS software. Calculations showthat area of a circular patch is less than area of a rectangular patch. The antennas were simulatedin MATLAB and results are tabulated. Results (Table 2) shows the comparison that circularpatch Yagi antenna radiates electric field towards broadside than rectangular patch and it offershigh directivity for frequency above 15GHz, around 3dB even though it has side lobes. The effectof different distances between reflector and driven element is almost negligible (from Table 3). Inboth the arrays, the performance is degraded due to high values of relative permittivity εr (fromTable 4) and the effect of substrate on the result is very less at low relative permittivity εr (fromTable 5). Finally rectangular patch yagi-array designed in Anasoft HFSS and compared withMATLAB results and observed 2dB difference. Referring to the Table 7 it is proved that both thepentagon & circle shapes acts as a perfect choice in the design of radio altimeter . This designoffers dual resonant frequency band with is exactly not available with the rectangular structure.With this innovative step there is an increase in the probability of utilization of the otherstructures of radio altimeter. This work can be extended for different Yagi designs for circularpolarization at different frequencies for different applications.

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